Referring to FIGS. 1 and 2, a first conventional punching device is adapted to cut a sheet material 40, and includes a punching head 51, a guiding seat 52, and a bottom seat 53 disposed below and connected to the guiding seat 52. The punching head 51 is movable relative to the bottom seat 53 between a standby position and a punching position. When the punching head 51 is at the standby position, the punching head 51 extends through the guiding seat 52, and is spaced apart from the bottom seat 53 such that, the sheet material 40 can be inserted into a space between the guiding seat 52 and the bottom seat 53. When the punching head 51 moves from the standby position to the punching position, the punching head 51 is inserted into a corresponding through hole of the bottom seat 53, and cooperates with the bottom seat 53 to cut the sheet material 40 disposed there between. The guiding seat 52 and the bottom seat 53 of the first conventional punching device must be formed as one piece, or be made of the same materials, so that the dimension of the guiding seat 52 can be precisely controlled, and the punching head 51 can be precisely guided by the guiding seat 52 to be inserted into the corresponding through hole of the bottom seat 53 when moving from the standby position to the punching position. After the sheet material 40 is cut, the punching head 51 moves away from the bottom seat 53 and returns to the standby position, and the sheet material 40 is restrained in the space between the guiding seat 52 and the bottom seat 53 by the guiding seat 52 to be prevented from being lifted with the return of the punching head 51 and getting stuck in the first conventional punching device.
As shown in FIG. 3, a second conventional punching device is disclosed in U.S. Pat. No. 7,201,101, and includes a bottom seat, a punching head, and a resilient member connected between the bottom seat and the punching head. The bottom seat has four upright guiding rods slidably engaging the punching head such that, the punching head can move along the guiding rods. Such configuration limits the second conventional punching device to be adapted to cut the corner 411 of a sheet material 41 away only, or to be adapted to form embossed patterns at the corner portions 411 of a sheet material only. When the second conventional punching device is attempted to cut holes 412 in a sheet material 41, since the sheet material 41 is not restrained between the bottom seat and a guiding seat as configured in the first conventional punching device, the sheet material 41 would be lifted by the return of the punching head which are used to cut the holes 412 and gets stuck in the second conventional punching device. In addition, the guiding rods form a blocking wall, such that the second conventional punching device is adapted to cut the corner portions 411 of the sheet material only.